The present invention is in the field of audio processing, especially spatial audio processing, and the merging of multiple spatial audio streams.
DirAC (DirAC=Directional Audio Coding), cf. V. Pulkki and C. Faller, Directional audio coding in spatial sound reproduction and stereo upmixing, In AES 28th International Conference, Pitea, Sweden, June 2006, and V. Pulkki, A method for reproducing natural or modified spatial impression in Multichannel listening, Patent WO 2004/077884 A1, September 2004, is an efficient approach to the analysis and reproduction of spatial sound. DirAC uses a parametric representation of sound fields based on the features which are relevant for the perception of spatial sound, namely the direction of arrival (DOA=Direction Of Arrival) and diffuseness of the sound field in frequency subbands. In fact, DirAC assumes that interaural time differences (ITD=Interaural Time Differences) and interaural level differences (ILD=Interaural Level Differences) are perceived correctly when the DOA of a sound field is correctly reproduced, while interaural coherence (IC=Interaural Coherence) is perceived correctly, if the diffuseness is reproduced accurately.
These parameters, namely DOA and diffuseness, represent side information which accompanies a mono signal in what is referred to as mono DirAC stream. The DirAC parameters are obtained from a time-frequency representation of the microphone signals. Therefore, the parameters are dependent on time and on frequency. On the reproduction side, this information allows for an accurate spatial rendering. To recreate the spatial sound at a desired listening position a multi-loudspeaker setup is needed. However, its geometry is arbitrary. In fact, the signals for the loudspeakers are determined as a function of the DirAC parameters.
There are substantial differences between DirAC and parametric multichannel audio coding such as MPEG Surround although they share very similar processing structures, cf. Lars Villemoes, Juergen Herre, Jeroen Breebaart, Gerard Hotho, Sascha Disch, Heiko Purnhagen, and Kristofer Kjrlingm, MPEG surround: The forthcoming ISO standard for spatial audio coding, in AES 28th International Conference, Pitea, Sweden, June 2006. While MPEG Surround is based on a time-frequency analysis of the different loudspeaker channels, DirAC takes as input the channels of coincident microphones, which effectively describe the sound field in one point. Thus, DirAC also represents an efficient recording technique for spatial audio.
Another conventional system which deals with spatial audio is SAOC (SAOC=Spatial Audio Object Coding), cf. Jonas Engdegard, Barbara Resch, Cornelia Falch, Oliver Hellmuth, Johannes Hilpert, Andreas Hoelzer, Leonid Ternetiev, Jeroen Breebaart, Jeroen Koppens, Erik Schuijer, and Werner Oomen, Spatial audio object coding (SAOC) the upcoming MPEG standard on parametric object based audio coding, in 124th AES Convention, May 17-20, 2008, Amsterdam, The Netherlands, 2008, currently under standardization in ISO/MPEG.
It builds upon the rendering engine of MPEG Surround and treats different sound sources as objects. This audio coding offers very high efficiency in terms of bitrate and gives unprecedented freedom of interaction at the reproduction side. This approach promises new compelling features and functionality in legacy systems, as well as several other novel applications.